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https://leetcode.com/problems/maximum-level-sum-of-a-binary-tree/description/
1161. Maximum Level Sum of a Binary Tree
返回最大的總和層數(每層)，如果總和相同取較小的。
Example 1:
https://assets.leetcode.com/uploads/2019/05/03/capture.JPG
Input: root = [1,7,0,7,-8,null,null]
Output: 2
Explanation:
Level 1 sum = 1.
Level 2 sum = 7 + 0 = 7.
Level 3 sum = 7 + -8 = -1.
So we return the level with the maximum sum which is level 2.
Example 2:
Input: root = [989,null,10250,98693,-89388,null,null,null,-32127]
Output: 2
思路：
1.用BFS去遍歷，每次算出來的都是當前層數和，如果當前和大於前面的才更新結果。
Java Code：
-----------------------------------------------
class Solution {
public int maxLevelSum(TreeNode root) {
int max = root.val;
int res = 1;
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
int level = 1;
while (!queue.isEmpty()) {
int size = queue.size();
int sum = 0;
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
sum += node.val;
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
if (sum > max) {
max = sum;
res = level;
}
level++;
}
return res;
}
}
-----------------------------------------------
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